An ideal HIV vaccine should stimulate strong, vigorous and durable HIV-specific humoral and cellular immune responses. A key component of such a vaccine will be the ability to stimulate high levels of HIV-specific CD4+ T helper responses. The evaluation of the immunogenicity of potential candidate vaccines is dependent on having rapid and efficient methods for the assessment of HIV-specific immune responses generated by these vaccines. Since classical cell culture methods for quantification of cellular immune responses, such as limiting dilution analysis, are very labor intensive, their use in testing potential vaccines is restricted. The assessment of CD4+ T helper responses has traditionally been measured by the incorporation of tritiated thymidine by proliferating cells in short term (3-4 days) cultures, with or without selected antigens. This crude method does not accurately quantify antigen specific CD4 cells. In this application, we aim to develop a rapid and quantitative method for assessing T helper cells in HIV-1 infected individuals. Specifically, we propose to construct tetrameric MHC class II and peptide complexes then apply them to the study of HIV-specific T helper cells. We have used same approach to quantify HIV-specific CTL responses with MHC Class I tetramers. The development of Class II tetramers will conceivably be applicable to large scale vaccine trials as well as the study of CD4 T helper responses in HIV-infected individuals.